A known pressure sensor, as shown in U.S. Pat. No. 4,875,135, comprises a capacitive pressure transducer having a thin ceramic diaphragm mounted in closely spaced, sealed, overlying relation to a ceramic base, and having metal coatings deposited on respective opposing surfaces of the diaphragm and the base to serve as capacitor plates arranged in predetermined closely spaced relation to each other to form a capacitor. The diaphragm, at a first face surface of the transducer, is arranged to be exposed to an applied fluid pressure to provide a capacitance which varies in response to changes in the applied pressure. Transducer terminals are disposed on the opposite, second, face surface of the transducer. A connector body formed of electrical insulating material has connector terminals mounted on the body and a metal housing secures the connector body in overlying relation to the second face surface of the transducer to form a chamber therebetween. An electrical circuit is disposed in the chamber electrically connected to the transducer and connector terminals for providing an electrical signal corresponding to the applied pressure.
The electrical circuit includes a flexible substrate which has one end portion disposed in overlying relation to the second face surface of the transducer and which mounts electrical circuit components in electrically connected relation to the transducer terminals and an opposite end portion folded over and in alignment with said one end portion. The opposite end portion mounts circuit paths thereon which are electrically connected to the connector terminals.
Signal conditioning circuits used with capacitive pressure transducers, as described, couple the transducer to a reference capacitor in a charge-lock loop relation having a common node for forming a capacitance-to-voltage converter circuit which is ratiometric with respect to supply voltage to vary output voltage within a selected range in response to variations in transducer capacitance in a corresponding range as set forth in commonly assigned U.S. Pat. No. 4,982,351. An array of switches cycles the voltage across the transducer and reference capacitors with opposite transitions with a predetermined frequency so that change in transducer capacitance results in a differential voltage at the common node. That voltage is then amplified or otherwise conditioned to drive a current-source sink network to adjust the sensor output voltage and, by a feedback path, to restore a balance condition at the node, thereby to maintain the sensor output voltage at a level related to the transducer capacitance.
Capacitive pressure transducers having ratiometric signal conditioning circuits as described above are in wide use in equipment incorporating A/D converters and sensors in networks having a reference that defines a given range. For example, the same 5 volt supply used to power the sensor is used for setting the range on the A/D converter so that changes in supply will affect both the converter and the sensors thereby desensitizing the network to such changes.
However, in an industrial environment, typically separate power supplies are employed for transducers which may be mounted at various different locations of a site. Such power supplies can vary widely, for example, between 8 and 36 volts. Although pressure responsive capacitive transducers having voltage regulators are known in the prior art, they are of relatively high cost and exhibit poor accuracy relative to their cost.